The 6870 is enjoying a bit of a renaissance; since it's been updated by the 69xx series cards, the 68xx nonetheless remains a very competitive card but most importantly, the price has dropped substantially. The "Barts" GPU core was intended to update the older 58xx series with a die-shrink, improved antialiasing, crossfire performance, and cooling. The 6870 is actually a replacement for the 5770, rather than the 5870, and fits in between the 5850 and 5870 in performance terms.
As a standalone card, these are pretty fast; around Geforce 460 levels, but the real secret is Crossfire performance. I picked up two of these, and reviews have found that running two 6870s in CrossfireX mode delivers scaling on the order of 85-99% - i.e. very close to literally twice as fast. Two of these cards in concert are £100 cheaper than a single Geforce 580, but importantly, faster. I've been trying out these cards in various recent games such as the Hard Reset demo, and found that when AMD released the updated Crossfire profile for the title, average framerates went from 55fps with a single card to 120fps with two cards (using an i5 2500K at 4.3Ghz). Also remember these are DX11 capable cards, with tesselation and morphological AA support. Despite running two cards instead of one, these cards are quiet and run around 70C at full load under e.g. Furmark. I initially had a bit trouble using the HDMI-out on these cards, entailing some swapping around etc. and was never able to resolve it, but the DVI ports work fine and all is well now.
These are decent cards individually, but the sweet spot right is two of them. It's worth experimenting.
NB : for some reason, Sapphire didn't provide a Crossfire bridge connector with either card, which was a bit of a bummer. I was able to find one at a local PC repair/building place, however.